Since a tire for a construction vehicle runs over rough ground surfaces, and at the same time, is subjected to a heavy load, a high tensile strength (breaking load) is required for steel cords used for reinforcing layer such as a carcass or a belt for a tire for the construction vehicle. Consequently, for such reinforcing layers, a steel cord with a so-called multi-twisted structure such as a 7×(3+9) structure or a 7×(3+9+15) structure in which a plurality of sheath strands are twisted around a core strand is generally used.
Further, since the tire for a construction vehicle frequently suffers a damage as a result of a heavy load while being in contact with a rough ground surface, such countermeasures have been taken, as use of a large diameter cord as a reinforcing cord, and use of a so-called high tensile strength steel to increase the tensile breaking force per unit area of the cord, so as to increase the tensile breaking strength in the axial direction of the cord.
However, a problem has been known with respect to a multi-twisted cord which is a plurality of twisted strands, that the cord strength is lower than the total of the strengths of the wires to such an extent as is unexplainable from the decrease attributable to the twist angle. To cope with the problem, for example, Patent Document 1 discloses a technique for avoiding premature breakage of a part of the wires to suppress the decrease of the strength in the axial direction of the cord.
Patent Document 2 discloses a technique for improving the cut resistant property of a steel cord for reinforcing rubber articles with a multi-twisted structure in which a plurality of sheath strands are twisted around a plurality of core strands placed in mutually parallel orientation, without increasing the rubber weight of a tire and without generating an early stage propagation of a crack between neighboring cords, by making the number of the core strands and the number of the sheath strand and the twist pitch of the sheath strands and the diameters of the sheath strands each satisfy a predetermined relation.
Further, Patent Document 3 discloses a technique for improving the cut resistant property of a steel cord with a multi-twisted structure in which sheath strands are twisted around core strands, without decreasing the strength of the axial direction of the cord, by making the ratio of the minimum diameter of the profile shape of the cord at the cross-section orthogonal to the cord axes to the maximum diameter thereof and the ratio of the diameter of the wire constituting the outermost layer of the sheath strand to the diameter of the sheath strand each in a predetermined range.